[time-nuts] Clock Correction Algorithm

Miguel Gonçalves mail at miguelgoncalves.com
Thu Nov 24 09:21:37 EST 2011

Hi Javier!

Thanks for your help. 

On 24/11/2011, at 13:16, Javier Herrero <jherrero at hvsistemas.es> wrote:

> El 24/11/2011 13:56, Miguel Gonçalves escribió:
>> Right! That is what I am doing.
>> The problem is the drift between adjustments. Arduino's clock is slow 25 ms every minute and if temperature changes it will surely be different.
> That is more than 400ppm error, that sounds quite high. Is the clock frequency of the Arduino so drifted? I would try first to check (and correct a bit) the source of that error. If the crystal frequency is ok, perhaps there is an error in the programming of the timer that generates the timing interrupt.

I am using an Arduino Uno that presumably is running at 16 MHz. 

I am using a 16 bit timer with a 256 pre-scaler:

16000000 / 256 = 62500 Hz

1 / 62500 = 16 us

65536 - 62500 = 3036

I am setting the counter to 3036 and let it overflow after 65535.

If I check the error every second I see the clock getting behind correct time and after 60 seconds it's 25 ms apart from UTC. 

Perhaps I should increase the initial value from 3036 to 3036+25E6/(60/16). I will make the second shorter this way. 

What do you think?


> Regards,
> Javier
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